Add an EventuallyChecker

[TeodorPt]

This PR proposes one form of an EventuallyChecker, which allows asserting eventually (i.e. retry for a given period of time until the assertion holds true) and also for stability (i.e. retry for a given period of time during which the assertion should still hold true).

The EventuallyChecker implements the Checker interface so it can be used like any regular checker. It expects as argument a function with no parameters and one return value.

See issue #121.

[TeodorPt]

@rogpeppe ptal when you have the chance, this is an implementation proposal for the Eventually checker you proposed